When differentiating cells are transferred to a fresh growth medium in which they would ordinarily be able to multiply, they continue their developmental process once they have reached a certain developmental stage. Using sporulation of Bacillus subtilis as a model system, it was shown by electron-microscopy that the morphological stage at which this commitment to continue differentiation occurs depends on both the original environment in which these cells multiplied and the environment in which they differentiate. The latest time of commitment coincides with the completion of a forespore double-membrane: i.e. a membrane configuration that does not permit active transport but only facilitated diffusion. Whereas cell growth is arrested by fusidic acid, bacilli become resistant to this inhibitor of protein synthesis at some time of development. The morphological stage of this resistance was identified as that of the prespore septum formation. Fusidic acid apparently interferes with septation as it also causes the formation of long non-separated cells.